STOCKHOLM -- Being overweight or obese poses as much of an increased risk of mortality in young adults as smoking, researchers here reported.

Action Points

Explain to interested patients that obese and overweight adults have the same risk of premature death as heavy and light smoking, respectively.

Note that the researchers found no interaction between body mass index (BMI) and smoking on risk.

Note also that this study cannot determine causality.

STOCKHOLM, Feb. 27 -- Being overweight or obese poses as much of an increased risk of mortality in young adults as smoking, researchers here reported.

Obese and overweight adolescents have the same risk of premature death as heavy and light smoking, respectively, Martin Neovius, Ph.D., of the Karolinska Institute, and colleagues reported online in BMJ.

"Obesity and overweight were as hazardous as heavy and light smoking, respectively," the researchers said, "but there was no interaction between BMI and smoking status."

Dr. Neovius said that both overweight and smoking are associated with increased risks of cardiovascular disease and certain cancers. Although the pathogenic mechanisms differ, he said, a synergistic effect of smoking and obesity may be possible.

Hypothesizing that both weight and smoking in late adolescence increase the risk of mortality, the researchers conducted a retrospective study of 45,920 Swedish men who had been followed for 38 years, beginning in 1969.

During the follow-up, 2,897 patients died.

The researchers found that obesity and overweight in late adolescence increased mortality risk, regardless of smoking status.

Similarly, mortality risk increased for those who smoked either one to 10 cigarettes per day (HR 1.55, 95% CI 1.41 to 1.70, P<0.001) or more than 10 cigarettes daily (HR 2.18, 95% CI 1.99 to 2.39, P<0.001) compared with nonsmokers.

However, the researchers found no interaction between BMI and smoking. While the combination of heavy smoking and obesity was associated with a large increase in risk of death, it was not significantly higher than either factor alone.

"Whether the combination of heavy smoking and obesity has synergistic effects will require further study as the point estimate in this study was large but did not reach statistical significance," the researchers said.

They also noted that they found a significantly increased risk of death in men with a BMI below 17 that was similar to the risk seen in overweight men. Previous studies have found a similar risk, the researchers said.

"Excess risk in the overweight and obese are to a large extent driven by increased mortality due to cardiovascular causes and cancers," Dr. Neovius said. "In the underweight, it is injuries and external causes that are overrepresented. [It] has previously also been shown that suicide is negatively associated with BMI."

Among the study's limitations were self-reported BMI data and its application to the population as a whole, since the researchers focused on men. They also could not exclude residual confounding caused by unmeasured confounders and thus could not determine causality.

Still, the researchers concluded that "overweight, obesity, and smoking among adolescents might be good targets for intensified public health initiatives."

The study was funded by the Arbetsmarknadens Forsakrings-och Aktiebolag (AFA), the Swedish Heart-Lung Foundation, and the Swedish Research Council.

The researchers reported no conflicts of interest.

Reviewed by Zalman S. Agus, MD Emeritus Professor University of Pennsylvania School of Medicine